Over the past decade, the technique of polymer thin film deposition known as “Layer-by-Layer” has proven its versatility in creating very uniform films of precisely controllable thickness, even on the nanometer length scale. This process is commonly used to assemble films of oppositely charged polyelectrolytes electrostatically, but other functionalities such as hydrogen bonding can be the driving force for film assembly. Typically, this deposition process involves the submersion of a substrate having an inherent surface charge into a series of liquid solutions, or baths. Exposure to the first polyion bath, which has charge opposite that of the substrate, results in charged species near the substrate surface adsorbing quickly, establishing a concentration gradient, and drawing more polyelectrolyte from the bulk solution to the surface. Further adsorption occurs until a sufficient layer has developed to mask the underlying charge and reverse the net charge of the substrate surface. In order for mass transfer and adsorption to occur, this exposure time is typically on the order of minutes. The substrate is then removed from the first polyion bath, and is then exposed to a series of water rinse baths to remove any physically entangled or loosely bound polyelectrolyte. Following these rinse baths, the substrate is then exposed to a second polyion bath, which has charge opposite that of the first polyion bath. Once again adsorption occurs, since the surface charge of the substrate is opposite that of the second bath. Continued exposure to the second polyion bath then results in a reversal of the surface charge of the substrate. A subsequent rinsing is then performed to complete the cycle. This sequence of steps is said to build up one ‘layer pair’ of deposition and can be repeated as desired to add further layer pairs to the substrate.
While this procedure as described is able to produce extremely uniform films as thin as one nanometer per layer pair, it is not uncommon that an individual layer pair may require upwards of thirty minutes to deposit. For a twenty-five layer pair film, the deposition process may then require more than twelve hours to complete. As a result, the Layer-by-Layer (LbL) dipping technique is typically carried out by a computer controlled slide-stainer to eliminate the need for human interaction. The choice of polyelectrolyte solvent is thereby typically limited to those solvents with relatively low vapor pressure, such as water, to avoid evaporation and species concentration during extended dipping periods.
Furthermore, since LbL is typically based on an electrostatic phenomenon, the degree of ionization of each polyelectrolyte in solution has a profound effect on the strength of interaction felt with the surface, and thus, the thickness of the adsorbed layer. For weak polyelectrolytes, pH has been most commonly used to vary charge density along the polymer chain and thus control layer thickness. For strong polyelectrolytes, charge shielding by varying ionic strength accomplishes the same function. In the case of an absorbent substrate, such as fabric, the cyclic nature of the dipping process can lead to a degree of carryover from the rinse baths to the subsequent polyelectrolyte solutions. This carryover can induce an observable change in the pH of the polyelectrolyte solutions, which may be unacceptable in certain cases. Additionally, long sample preparation times can allow the pH of the polyelectrolyte solution to drift, as evaporation and concentration of the solution occurs.
In an effort to eliminate rinse water contamination, robotic modifications have been made to dipping systems. One such modification involves spraying the sample with water, which immediately drains away. Since the contaminated water drains away, and is not left in the rinse bath, the likelihood of contamination and swelling of the film between alternating depositions is lessened.
As stated above, the traditional Layer by Layer process is very time consuming. In the dipping method, polymer chains must diffuse to the charged surface once a depletion layer is developed by adsorption of nearby molecules. Thus, there exists a diffusion time scale, which is inversely proportional to the diffusivity of the polyelectrolyte through the solvent, limiting the rate of deposition.
This characteristic time increases with decreasing diffusivity values, commonly seen with larger molecular weight molecules.
A less time consuming method of performing layer by layer deposition is needed. However, such a method cannot reduce the quality or uniformity of the layers that are deposited, since these factors are critical. Additionally, methods performing layer by layer deposition on substrates having large surface areas or three dimensions are also required.